Stress testing has not been widely applied to pediatric patients with heart disease, and their physical working capacity is estimated largely from clinical information and hemodynamic data at rest. Since many such patients are now reaching adolescence and adulthood due to improved medical and surgical care, there is a demonstrated need to find realistic answers to questions of physical work capacity, employability, insurability, participation in sports, and quality of life. It is the broad objective of the proposed study to approach these problems by systematic study of exercise performance, oxygen transport capacity and the cardio-respiratory responses (ventilation, 02-C02 gas exchange, ECG, heart rate, stroke volume, and a-v 02 difference) to standardized submaximal and maximal cycle ergometer exercise, and by investigation of the effect of physical conditioning of these responses. Specifically we propose to study initially three patient groups who have undergone intracardiac repair of their defects: tetralogy of Fallot, transposition of the great arteries, and left ventricular outflow obstruction. The data will be correlated with clinical, surgical and anatomic findings, hemodynamic data, and normal standards, and analyzed to assess the degree of functional repair and the effects of residual lesions and differences in surgical technique on exercise performance. Our experience in children with congenital heart disease indicates that successful, routine application of exercise studies in these patients requires non-invasive methods which make minimal demand on patient cooperation. In order to meet these requirements we have been concerned with the development of non-invasive methods which permit quantitative assessment of: the ECG, heart rate, ventilation and respiratory gas exchange (O2-CO2) on a breath-by-breath basis, pulmonary blood flow and the systemic a-v O2 difference. The engineering aspects of the proposed study are concerned with the integration of these methods into a system (hardware, software) for on-line analysis with emphasis on simplicity of operation and high data accuracy and reliability.